ON TKOF ROLL BOTH ENG BLEED VALVES FAILED TO THE EMER POS.

Narrative

AFTER A PART 91 STAGE LEG XXX-ZZZ TO PICK UP 6 PAX FOR PART 135 TRIP ZZZ-BBB; THE LEG TO BBB WAS DELAYED APPROX 1 HR DUE TO WX BEING BELOW MINIMUMS AT BBB. WX LIFTED; ACFT STARTED TAXI TO RWY 5 AT ZZZ. CHKLISTS/BRIEFS CONDUCTED; ALL NORMAL. CLRED FOR TKOF RWY 5; PWR APPLIED THEN NOTICED AIR NOISE OF PRESSURIZATION LOUDER THAN NORMAL. PWR THEN WAS BROUGHT BACK TO IDLE. PLT AND I TALKED ABOUT IT AND BLEED AIR/CABIN AIR VALVES RESET. TKOF PWR APPLIED AGAIN; ALL SEEMED/SOUNDED NORMAL. DURING TKOF ROLL THE BLEED VALVES WENT INTO EMER POS. AFTER GEAR RETRACTION LIGHT SMOKE WAS FILLING THE COCKPIT/CABIN; SMELLED LIKE JET ENG EXHAUST. ALSO HEAT WAS FELT AS THIS IS NORMAL WHEN THE SYS GOES INTO EMER MODE. THE SMOKE WAS OF CONCERN. IT WAS NEVER HVY ENOUGH TO DON OXYGEN MASK OR GOGGLES. ABNORMAL CHKLIST ACCOMPLISHED; SHUT OFF BOTH BLEEDS AND CABIN AIR. DECLARED AN EMER AND RETURNED TO LAND AFTER DUMPING APPROX 800 LBS OF FUEL. WE SPOKE TO OUR MAINT DEPT AND RECYCLED VALVES. AT THIS POINT ALL PAX WERE UNLOADED AND THE PART 135 LEG OF ZZZ-BBB WAS CANCELED. ACFT WAS FLOWN BACK TO XXX PART 91 WITH NO PROBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ANALYST CONTACTED THE RPTR WHO RELATED THE FOLLOWING NARRATIVE. AFTER RETURN TO THE FIELD I CONTACTED OUR OPS BASE MAINT AND WAS ADVISED TO RETURN WITH THE BLEED VALVES AND AIR-CONDITIONING IN OFF POS. RETURN FLT WAS MADE WITH BLEEDS AND AIR-CONDITIONING OFF. I AND PF DID NOT UNDERSTAND WHAT CAUSED BOTH BLEED VALVES TO FAIL TO THE EMER POS. I HAVE NO KNOWLEDGE OF ANY MAINT PERFORMED OR COMPONENTS REPLACED TO CORRECT THE BLEED PROB. THE ACFT MANUFACTURER'S REPRESENTATIVE ADVISED ME THE LIGHT SMOKE OR VAPOR WAS RESIDUAL DUST OR LIGHT OIL COATING IN THE PNEUMATIC DUCTS. THERE WAS NO PREVIOUS HISTORY ON THIS AIRPLANE OF PNEUMATIC FAILURE. THE FAA DDI NOT CONTACT THE RPTR.